Wisconsin Maximum Research Articles

Leadership and Self‐Efficacy: Lessons from Harriet Tubman

Leadership and Self‐Efficacy: Lessons from Harriet Tubman

Objective Reconstructions of the Late Wisconsinan Laurentide Ice Sheet and the Significance of Deformable Beds

A three dimensional steady state plastic ice model; the present surface topography (on a 50 km grid); a recent concensus of the Late Wisconsinan maximum margin (PREST, 1984); and a simple map of ice yield stress are used to model the Laurentide Ice Sheet. A multi-domed, asymmetric reconstruction is computed without prior assumptions about flow lines. The effects of possible deforming beds are modelled by using the very low yield stress values suggested by MATHEWS (1974). Because of low yield stress (deforming beds) the model generates thin ice on the Prairies, Great Lakes area and, in one case, over Hudson Bay. Introduction of low yield stress (deformabie) regions also produces low surface slopes and abrupt ice flow direction changes. In certain circumstances large ice streams are generated along the boundaries between normal yield stress (non-deformable beds) and low yield stress ice (deformabie beds). Computer models are discussed in reference to the geologically-based reconstructions of SHILTS (1980) and DYKE ef al. (1982).

Glacial and Postglacial History of the White Cloud Peaks-Boulder Mountains, Idaho, U.S.A.

Glacial and glaciofluvial deposits are mapped and differentiated to develop new local, relative-age (RD) stratigraphies for the North Fork of the Big Lost River, Slate Creek and Pole Creek drainages in the White Cloud Peaks and Boulder Mountains, Idaho. This stratigraphic model expands the areal extent of the "Idaho glacial model". Volcanic ash samples collected from the study area are petrographically characterized and correlated, on the basis of mineralogy and glass geochemistry, to reference samples of identified Cascade Range tephras. Four distinct tephras are recognized including; Mount St. Helens-Set S (13,600-13,300 yr BP), Glacier Peak-Set B (11,250 yr BP), Mount Mazama (6600 yr BP) and Mount St. Helens-Set Ye (4350 yr BP). A core of lake sediments containing two tephra units was obtained from a site called "Pole Creek kettle". Pollen and sediment analyses indicate three intervals of late Pleistocene and Holocene climatic change. Cool and wet climatic conditions prevailed in the region shortly before and immediately following the deposition of the Glacier Peak-Set B ash (11,250 yr BP). Climatic warming occurred from approximately 10,500 to 6600 yr BP after which warm, dry conditions prevailed. Sediment accumulation in the kettle ceased by 4350 yr BP. The presence of Glacier Peak-Set B tephra in the base of the Pole Creek kettle core provides a minimum age of 11,250 yr BP for the retreat of valley glaciers from their Late Wisconsinan maximum position. A radiocarbon date of 8450 + 85 yr BP (SI-5181), and the presence of Mount Mazama ash (6600 yr BP) up-core support the Glacier Peak-Set B identification.

Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum

Prior to 1943 the Laurentide Ice Sheet was considered to have three major domes centered in Keewatin, Labrador, and Patricia (TYRRELL, 1898 a, b; 1913). FLINT (1943) argued that these centres were of only local and temporary importance and favoured a single-domed ice sheet. Despite the lack of supporting geological evidence, and despite the proposition of a Foxe Dome in the interim (IVES and ANDREWS, 1963), the single-dome concept was not seriously challenged until the late 1970's and, in fact, is still strenuously supported (HUGHES era/., 1977 ; DENTON and HUGHES, 1981). This paper extends and modifies recent conclusions that the Laurentide Ice Sheet had more than one dome at the Late Wisconsin maximum. We propose a model incorporating five domes (M'Clintock, Foxe, Labrador, Hudson, and (?) Caribou) based on the position of ice divides, ice flow patterns, drift composition, late-glacial features, postglacial isostatic recovery and free-air gravity anomalies. Our Labrador and Hudson domes closely correspond to Tyrrell's Labradorean and Patrician ice sheets; our Caribou and M'Clintock domes together with the Franklin Ice Complex over the Queen Elizabeth Islands north of the Laurentide Ice Sheet, correspond to Tyrrell's original Keewatin Ice Sheet. The style of glaciation of the Foxe Basin region was not known to Tyrrell, but our reconstruction of the Foxe Dome is in close agreement with the original proposal of Ives and Andrews. Like Tyrrell, our reconstruction is based on field evidence obtained through extensive mapping; the single dome model continues to be unsupported by geological data.

The Laurentide and Innuitian ice sheets during the Last Glacial Maximum

The Late Wisconsinan advance of the Laurentide Ice Sheet started from a Middle Wisconsinan interstadial minimum 27–30 14C ka BP when the ice margin approximately followed the boundary of the Canadian Shield. Ice extent in the Cordillera and in the High Arctic at that time was probably similar to present. Ice advanced to its Late Wisconsinan (stage 2) limit in the northwest, south, and northeast about 23–24 14C ka BP and in the southwest and far north about 20–21 14C ka BP. In comparison to some previous reconstructions of ice extent, our current reconstruction has substantially more Late Wisconsinan ice in the High Arctic, where an Innuitian Ice Sheet is generally acknowledged to have existed, in the Atlantic Provinces, where ice is now thought to have extended to the Continental Shelf edge in most places, and on eastern Baffin Island, where ice probably extended to the fiord mouths rather than to the fiord heads. Around most of the ice margin, the Late Wisconsinan maximum ice extent either exceeded the extent of earlier Wisconsinan advances or it was similar to the Early Wisconsinan advance. Ice marginal recession prior to 14 14C ka BP occurred mainly in deep water and along the southern terrestrial fringe. However, Heinrich event 1 probably drew down the entire central ice surface at 14.5 14C ka BP sufficiently to displace the Labrador Sector outflow centre 900 km eastward from the coast of Hudson Bay. The onset of substantial ice marginal recession occurred about 14 14C ka BP in the northwest, southwest, and south but not until about 10–11 14C ka BP in the northeast and in the High Arctic. Thus, the period of maximum ice extent in North America generally encompasses the interval from ∼24/21 to 14 14C ka BP, or considerably longer than the duration of the LGM defined as occurring during a period of low global sea level as well as during a time of relative climate stability ∼18 14C ka BP. The interval of advance of much of the Laurentide Ice Sheet to its maximum extent (between ∼27 14C ka BP and ∼24 14C ka BP) coincides with a suggested interval of rapid fall in global sea level to near LGM levels.

Wind-Transverse Corrugations in Pleistocene Periglacial Landscapes of Central Pennsylvania

Periglacial conditions beyond the Wisconsin glacial limit produced wind-transverse corrugations in east-central Pennsylvania. The corrugations are low, linear ridges typically several hundred meters long, 10–30 m wide, and 3–8 m above the local slope. They are composed of matrix-supported diamictons. Depending upon landscape position, the corrugations form welts, step and risers, or ground-ice-scar ramparts. The average direction normal to the corrugations is clustered at 118°/298°, similar to the late Wisconsin or early Holocene wind direction of 284° measured from a nearby fossil dune field. But the corrugations were neither eroded nor deposited by wind. Rather, they record nivation beside snow that accumulated in wind-transverse patterns. Their wide distribution implies that the region was treeless during the late Wisconsin maximum. Present discontinuities in these and other periglacial features suggest wasting of ice-rich fill in upland valleys, with consequent widening of stream channels and fan growth, at the end of periglacial time.

Latest pleistocene and early Holocene fluctuations of glaciers in the Canadian and northern American Rockies

In the Canadian and northern American Rockies small-scale fluctuations of glaciers during and after late Wisconsinan deglaciation have been identified using stratigraphy of superimposed glacial deposits, geomorphology, and records of glacial advance in lake and peat sediments. Several authors have proposed that valley-glacier readvances of relatively modest scale followed the late Wisconsinan maximum. Evidence for some of these is difficult to evaluate from the literature and no deposits recording such readvances have been numerically dated. Several minimum-limiting radiocarbon dates for deglaciation suggest ice had retreated to within a few tens of km of present limits by ca. 12 ka BP, and to very near present limits by ca. 11 ka BP. One or more minor glacial advances, or stillstands during recession, evidently occurred in valley heads prior to 11 ka BP. Part of the evidence consists of moraines a few km downvalley from subsequently emplaced Little Ice Age and Crowfoot moraines. The Piper Lake moraine in the Mission Mountains of Montana is overlain by Glacier Peak G tephra, and thus has a minimum age of 11,200 14C years BP. The Crowfoot Advance, a minor advance of about the same magnitude as Little Ice Age advances, occurred between ca. 11,300 and 10,000 14C years BP and is thought to be related to the Younger Dryas climatic reversal. Crowfoot moraines have been found from Jasper National Park on the north to the Mission Mountains on the south. Crowfoot ELAs were an average of 40 m lower than modern ELAs around the Canada-U.S. border. There have been past proposals of significant early Holocene glacial advances in the Rockies, i.e. after Crowfoot time but before the LIA, but recent evidence indicates that glaciers then were less extensive than at present.

Eastern Mississippi delta: late Wisconsin unconformity, overlying transgressive facies, sea level and subsidence

Analysis of 92 engineering core logs located in the Balize sector of the eastern Mississippi delta focuses on the late Wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon. This major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin. Observations emphasize lithofacies distributions of the latest Pleistocene sediments underlying the unconformity, the late Pleistocene to early Holocene transgressive facies, and the immediately overlying deltaic deposits. Maps and a cross-section compiled with this information highlight the critical relation between lithofacies distributions and late Wisconsin sea-level oscillations. Core analysis reveals that the transgressive facies comprises distinct environments of deposition, offshore to onshore. Findings identify criteria to determine the approximate position of the shoreline at the late Wisconsin maximum sea-level lowstand and at the extent of early Holocene maximum marine inundation in the Balize complex. Mapping of lithologies along the late Wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high-resolution seismic profiles. An estimate of long-term averaged land subsidence suggests lowering of at least 1 mm/year near the shelfedge during the past ∼ 18 000 years. This mapping also refines sea-level response models for the Mississippi delta, including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other Recent marine deltas and in older, stacked deltaic sequences.

An ultrahigh resolution 3-D survey of the shallow subsurface on the continental shelf of New Jersey

Regional seismic surveys have identified a wedge of late Quaternary sediment extending 150 km south from the Hudson River apron along the edge of the continental shelf off New Jersey. The bottom of the sediment wedge is defined by a prominent reflector that is assumed to be an erosional surface carved during a lowstand of sea level, probably corresponding to the Wisconsin Maximum glaciation (about 18 000 years ago). An extremely high resolution 3-D seismic reflection survey of the southern part of the wedge (Figure 1) was carried out in 1993 as part of the STRATAFORM (STRATA FORMation on Margins) initiative, funded by the U.S. Office of Naval Research.

Late Wisconsin glacial advance and retreat patterns in southwestern Ohio, USA

Despite the application of radiocarbon dating for more than three decades along the southern margin of the Laurentide Ice Sheet, fundamental questions about the timing of glacial advances remain. For one of its sublobes, the Miami, we undertook areal mapping, detailed lithostratigraphic analysis, and radiocarbon dating to interpret four pulses of ice advance. On top of the undated sediments deposited during the first advance is a major unconformity. The second advance occurred about 20,000 BP and marks the beginning of the late Wisconsin glaciation. A minor recession (more than 30 km) ensued, but plants did not reoccupy the landscape. A third advance of the ice margin produced a stone‐rich lodgement till to within 20 km of the late Wisconsin maximum. The final ice motion only occurred in the northern part of the study area and may be of local extent. Large accumulations of supraglacial gravity flowtills and outwash mark the final ice‐margin retreat. Of these. only the second advance is well dated. This study implies that the number of advances of the ice margin is fewer than previously suggested. Consequently we argue that several of the sublobes across the southern margin of the Laurentide Ice Sheet acted in unison for the interval of 22.000 to 18.000 BP implying ice‐sheet external forcing.

Late Quaternary sedimentation off New Jersey: New results using 3-D seismic profiles and cores

Regional seismic reflection surveys have delineated a wedge of late Quaternary sediment extending 150 km south from the Hudson apron along the edge of the continental shelf off New Jersey. The bottom of the sediment wedge is defined by a prominent reflector (R), assumed to represent an erosional surface carved during a low-stand of sea level, probably corresponding to the Wisconsinan Maximum glaciation. A 3-D reflection survey of a 5 × 0.5 km area of the southern part of the wedge was carried out in October 1989. Line spacing was 10 m and shot spacing 2.5 m. Navigation achieved position accuracies to < 5 m. A series of cores both within and near the survey area yielded sedimentological and faunal data for ground-truth of the seismic results. The survey shows that the outer shelf sediment wedge has a complex internal structure unrelated to the present seafloor morphology. The sedimentary sequence appears to have been formed by a series of depositional events related to glacial melting and interrupted by at least one erosional episode, marked by a seismic surface that defines a system of meandering channels. Faunal studies of samples from cores indicate that the sediments were deposited at shallower depths than at present, but there is no evidence for deposition in shallower water or closer proximity to land than the modern mid-shelf. It appears that the history of periglacial marine sedimentation which accompanied the post-Wisconsinan deglaciation is no less complex than its continental counterpart.

Late Quaternary glacial and sedimentary history of Bonavista Bay, northeast Newfoundland

High-resolution seismic profiles and core data from Bonavista Bay, northeast Newfoundland, show that the sedimentary package above acoustic basement was deposited during the deglaciation of the Late Wisconsinan ice sheet. During the Late Wisconsinan maximum (~ 20 000 BP), Bonavista Bay was occupied by a grounded ice sheet that extended farther offshore. Deglaciation of the bay was rapid and occurred prior to ~ 13 500 BP during a time of rising sea level. A basal till was deposited beneath the grounded ice, and after lift-off an ice shelf developed over the outer basins where diamicton was deposited. The inner bay gradually deglaciated as the ice margin retreated to the present-day shoreline by ~ 13 000 BP, and fine-grained outwash sediments transported by interflows were rapidly deposited. The inner bay remained under the influence of one or more remnant ice centres until ~ 10 000 BP, with ice positioned on the Bonavista and Gander peninsulas. Normal marine conditions were established in the outer bay by ~ 13 500 BP and in the inner bay and fjords by ~ 10 000 BP.

Drumlin formation: a time transgressive model

Graphical and numerical reconstructions of the Rainy and Superior lobes of the Laurentide Ice Sheet suggest that drumlin formation was time transgressive. Suites of glacial landforms including drumlins, tunnel valleys, eskers, and ice‐collapse features can be correlated with specific recessional ice margins and are used as boundary conditions in the modeling. A contour map of the ice surface is then drawn using a specified basal shear stress. The shear stress can be constant or allowed to vary with position on the bed and is chosen to be consistent with the subglacial regime indicated by field evidence. Assuming that ice flow is parallel to drumlin orientations and perpendicular to the ice surface contours and moraines, the trend of drumlin axes is best accommodated by time transgressive drumlin formation during minor stillstands in the overall ice recession. The alternative, that drumlins were formed while the ice was at the Late Wisconsin maximum limit, requires large spatial variations in the basal shear stress distribution and therefore implies large mass‐balance gradients or large variations in basal sliding velocities over small distances, for which there is little evidence.

Sensitivity Experiments with a Model of the Ice Age Cycle: The Response to Milankovitch Forcing

Abstract We address the question of the extent to which a simple one-dimensional time-dependent climate model is able to explain the fluctuations of continental ice volume which have occurred during the last quarter of the Pleistocene epoch. The model includes explicit ice physics based on the Glen flow law and explicit glacial isostatic adjustment based on a realistic viscoelastic field theory. When the model is forced by the Milankovitch variations of insulation it delivers ice volume fluctuations which are very similar to those implied by the specmap δ18O record. In particular, the model appears capable of explaining the observed correlation between eccentricity and ice volume variations, the dominance of the 105 year period in the power spectrum of the ice volume time series, and the thickness and latitudinal extents of the continental ice sheets which existed at Wurm–Wisconsin maximum 18 000 years ago.

LINKAGE DISEQUILIBRIUM AND A CONTACT ZONE IN PLETHODON CINEREUS ON THE DEL‐MAR‐VA PENINSULA

A contact zone between two electrophoretically differentiated forms of Plethodon cinereus extends across the Del-Mar-Va Peninsula. Based on the presumed nature of regional climates during the late Pleistocene, it is suggested that the climate was inhospitable for P. cinereus on the Del-Mar-Va Peninsula at the Wisconsin maximum. It is postulated that, after the Laurentide ice sheet receded about 14,000 years BP, the peninsula was invaded by populations of P. cinereus from the north and south. When the two forms of P. cinereus met, a secondary zone of contact was produced. This contact zone is now apparent in an area in which six electrophoretic loci have concordant clinal changes in allele frequency. The lack of evidence for heterozygote deficiencies, the broad width of the contact zone, and the overall symmetry of introgression suggest that fusion of the populations is occurring freely. Two esterase loci are in linkage disequilibrium in four of eight populations to the north of the contact zone. As all populations of P. cinereus studied to date have disequilibrium coefficients of similar sign, selection is likely important in the maintenance of this linkage disequilibrium. Within the contact zone, the incidence of the linkage disequilibrium increases, but exchange of genes between the two introgressing populations may account for the higher incidence of linkage disequilibrium.

Summary of Quaternary Stratigraphy and history, Eastern Canada

Deposits of three Wisconsinan substages, Sangamonian Stage, and older Quaternary stratigraphic units are recognized in Eastern Canada. The age assignment of these units is based on radiocarbon dating and correlation of events. Quaternary deposits older than Sangamonian are recognized locally in Eastern Canada. In southern Ontario glacial deposits directly underlie Sangamonian sediments and are referred to as Illinoian in age. In other areas the ages of older sediments are largely unknown. Offshore core stratigraphy suggests that a major glaciation took place about 436 ka and that the Illinoian (oxygen isotope stage 6) was also a time of extensive glaciation. In this report Sangamonian is used as the name for the chronostratigraphic stage that includes all of deep-sea oxygen isotope stage 5 and consequently, on a regional basis, it includes warm interglacial deposits, glacial deposits and cool interglacial deposits. In southern Ontario the warm interglacial deposits are represented by the Don Formation, the stadial deposits by the Scarborough Formation and the cool interglacial deposits by the Pottery Road Formation. Warm interglacial deposits have not been recognized in Quebec (unless they are part of the pre-Johnville Sediments); the Becancour Till is included as glacial Sangamonian sediments, and the St. Pierre Sediments are recognized as cool interglacial sediments. The Early Wisconsinan appears to have been the time of maximum Wisconsinan glaciation in Eastern Canada with ice moving south of the International Boundary and well out onto the continental shelf. The Middle Wisconsinan was primarily a nonglacial period in southern Ontario and a glacial stade elsewhere in Eastern Canada. In southern Ontario the Middle Wisconsinan record has been subdivided into two interstades (Port Talbot and Plum Point), separated by a stade (Cherrytree). The Port Talbot Interstade began before the limit of radiocarbon dating (before 48 ka) and ended about 40 ka; glacial or near glacial conditions of the Cherrytree Stage lasted from about 40 to 35 ka ago, and the Plum Point Interstade was from about 35 to 23 ka ago. Central St. Lawrence Lowland was occupied by ice throughout the Middle Wisconsinan, but southeastern Quebec and the Montreal area were briefly deglaciated. Scattered evidence in Atlantic Canada suggests local deglaciation of coastal areas during Middle Wisconsinan but extensive ice remained on the continental shelf and ice from centres located on the shelf flowed onto land in at least two areas. Glacial conditions predominated throughout Eastern Canada during the Late Wisconsinan. At the Late Wisconsinan maximum, through-moving ice deposited the Catfish Creek Drift in southern Ontario but ice lobes, which developed in the basins of the Great Lakes after 15.5 ka, controlled ice flow during a period of ice margin oscillation and retreat. A calving bay developed in lower St. Lawrence valley, after the Late Wisconsinan maximum, causing a reversal of flow on the south shore of the St. Lawrence and replacing ice in the valley with the Champlain Sea about 12 ka. Late Wisconsinan glaciers were largely limited to land areas in Atlantic Canada. Local ice caps dominated with complicated patterns of flow and retreat developing as centres of accumulation shifted and competing ice centres achieved dominance. The period of Late Wisconsinan retreat in Atlantic Canada appears to have lasted from about 14 to 10 ka.

Sensitivity Experiments with a Model of the Ice Age Cycle: The Response to Harmonic Forcing

Abstract We address the question of the extent to which a simple one-dimensional time-dependent climate model is able to explain the fluctuations of continental ice volume which have occurred during the last quarter of the Pleistocene epoch. The model includes explicit ice physics based on the Glen flow law and explicit glacial isostatic adjustment based on a realistic viscoelastic field theory. When the model is forced by the Milankovitch variations of insulation it delivers ice volume fluctuations which are very similar to those implied by the specmap δ18O record. In particular, the model appears capable of explaining the observed correlation between eccentricity and ice volume variations, the dominance of the 105 year period in the power spectrum of the ice volume time series, and the thickness and latitudinal extents of the continental ice sheets which existed at Wurm–Wisconsin maximum 18 000 years ago.

Reply to Comments by J.T. Andrews on “Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum”

Reply to Comments by J.T. Andrews on “Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum”

Comments on “Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum”, by A.S. Dyke, L.A. Dredge and J.-S. Vincent

Comments on “Configuration and Dynamics of the Laurentide Ice Sheet During the Late Wisconsin Maximum”, by A.S. Dyke, L.A. Dredge and J.-S. Vincent. Un article de la revue Géographie physique et Quaternaire (Volume 37, numéro 1, 1983, p. 3-123) diffusée par la plateforme Érudit.

The Late Pleistocene and Holocene Fauna and Flora of Vulture Cave, Grand Canyon, Arizona

The Late Pleistocene and Holocene Fauna and Flora of Vulture Cave, Grand Canyon, Arizona